1. Field of the Invention
The present invention relates to an imaging optical system such as an imaging lens or a projector projection lens, and more particularly, to an imaging lens which is appropriate for a wide-angle lens having a wide angle of view.
2. Description of the Related Art
In the related art, when a wide landscape is to be photographed by a camera, or when a wide screen is to be projected within a short distance by a projector, a wide-angle lens having a wide angle of view is used. A wide-angle lens can be used for an apparatus where a distance between a reduction-side imaging plane and a lens end closest to the reduction-side imaging plane is long. For example, a single-lens reflex camera or a projector having a color combining system. For such applications, a wide-angle lens will typically use a retrofocus-type lens unit. The retrofocus-type lens unit is a lens unit with a lens having a strong negative refractive power disposed nearer to an enlargement-side imaging plane than a stop. Hereinafter, the reduction-side imaging plane side is referred to as the reduction-side; and the enlargement-side imaging plane side is referred to the enlargement-side.
However, as the retrofocus-type lens unit has a wider angle of view, the diameter of the enlargement-side lens is greatly increased. Techniques for solving the problem are discussed in the English Abstract of Japanese Patent Application Laid-Open No. 04-356008 and U.S. Patent Application Publication No. 2005/0117123.
The English Abstract of Japanese Patent Application Laid-Open No. 04-356008 discusses an optical system which forms an intermediate image of an object within a lens unit and re-forms the intermediate image on an image plane. Hereinafter, in order to avoid confusion in the specification, with respect to an in-lens conjugate point where the intermediate image is formed within a lens unit as a division point, the enlargement-side lens unit is referred to as a first lens unit, and the reduction-side lens unit is referred to as a second lens unit.
The first lens unit in the lens unit discussed in the English Abstract of Japanese Patent Application Laid-Open No. 04-356008 forms a reduced image of the object as an intermediate image by a reduction optical system. The second lens unit is configured as a relay system which forms the intermediate image on the image plane (an imaging plane of an image sensor). Accordingly, a back focus of the first lens unit is shortened, so that the diameter of the enlargement-side lens of the first lens unit is reduced.
The lens unit discussed in U.S. Patent Application Publication No. 2005/0117123 is a projection lens for a projector, which forms an image obtained by a light modulation element as an intermediate image and enlarges the intermediate image to project the enlarged image onto a projection receiving surface. Therefore, similarly to the English Abstract of Japanese Patent Application Laid-Open No. 04-356008, the diameter of the enlargement-side lens of the first lens unit is also reduced.
The English Abstract of Japanese Patent Application Laid-Open No. 2001-23887 discusses a projection optical system of an exposure apparatus, which forms an intermediate image, although the projection optical system is not a wide-angle lens.
However, the lens unit discussed in the English Abstract of Japanese Patent Application Laid-Open No. 04-356008 is a fisheye lens, and large distortion remains on a final image plane. Therefore, the lens unit is not appropriate for a wide-angle lens for general picture photographing or a projection lens for a projector, where distortion needs to be sufficiently corrected.
On the other hand, in the lens unit discussed in U.S. Patent Application Publication No. 2005/0117123), although distortion is corrected, aberration correction is independently performed in the first and second lens units disposed with respect to the in-lens conjugate point as a division point. Therefore, although the diameter of the enlargement-side lens is reduced, the total lens length is increased. In other words, both of the English Abstract of Japanese Patent Application Laid-Open No. 04-356008 and U.S. Patent Application Publication No. 2005/0117123 do not simultaneously accomplish the correction of distortion and the miniaturization in the optical axis direction.